1. Field of the Invention
This invention relates to a digital image output apparatus for forming a multi-gradation image, and more particularly to a multi-color, multi-gradation digital image output apparatus including a unit for controlling a toner supply amount so that the toner density or toner amount at the developers of the respective colors can be maintained at an optimal value.
2. Discussion of the Related Art
The binary developer, composed of toner and its carrier, that is used in electrostatic copying machines or electrostatic printers of the electrophotographic type impairs the quality of images when the toner density, i.e., the mixing ratio of the carrier and the toner, is altered. Thus, it is necessary to maintain the toner density in the developer at a constant irrespective of output conditions such as the number of output copies and output image density. Further, when a one-component developer is used, the amount of developing agent at the developer, i.e., the amount of toner, must be maintained at a constant. For this reason, a toner supply amount control unit is used so that the amount of toner to be supplied to the developers can be adjusted by controlling toner dispensing motors in accordance with the density of the image to be output which is detected in advance.
In prior art optical electrostatic copying machines, the control of toner supply to a developer involves prescanning a document, checking an average density using an optical sensor, and controlling the toner dispensing motors in accordance with the obtained average density
This type of toner supply amount control unit, involving the prescanning of a documents, uses the average density of the document as a control parameter. However, the average density is not in strict correspondence with the amount of toner to be consumed, and as a result, the toner supply amount obtained based on the average density is not optimal.
Also, of recent use are digital copying machines in which a document image is read by a CCD (charge coupled device) sensor. In such digital copying machines, the read signals are converted to digital video signals, and the image is output after subjecting the digital video signals to a predetermined image processing. In order to use the above-described toner supply amount control unit in such digital copying machines, a document density detection sensor is required in addition to the regular image reading CCD sensor, thereby making the machines expensive.
Thus, a toner supply amount control unit for the prior art digital copying machines, digital printers, etc. is designed to control the toner supply amount by recognizing that the image data contains density data of the document image.
That is, the digital image output unit counts the black bits in serially transmitted image data, e.g., the number of high-level bits, and the dispensing motors are controlled based on the obtained counts. Generally, the image data applied to the digital image output unit is binary data in which density data amount per pixel is 1 bit. Therefore, the number of bits substantially corresponds to the density, thereby allowing the toner density to be properly adjusted as long as the toner supply amount is controlled in accordance with the number of bits.
However, if this method is applied to an image output apparatus outputting multi-valued image data, e.g., gradation data having density data amount per pixel of 2 bits, accurate toner supply amount control can not be performed
The reason is as follows. For example, if the density data amount per pixel consists of 2 bits, gradation data takes four values corresponding to respective densities as shown in Table 1. Here, it is supposed that the relationship "white"&lt;"gray 1"&lt;"gray 2"&lt;"black" exists.
TABLE 1 ______________________________________ Density Data ______________________________________ White 00 Gray 1 01 Gray 2 10 Black 11 ______________________________________
If such data is transmitted serially and the number of highlevel bits, i.e., data "1," is counted, the results are 1 for "gray 1"; 1 for "gray 2"; and 2 for "black." Unlike the binary data, these counts have no correspondence with the toner consumption amount. For example, if the densities of contiguous pixels are "white," "gray 1," "gray 1," and "black," the image data of this particular portion of pixels is expressed as "00010111" and the bit count becomes 4. However, in the case of an array of pixels consisting of "white" "gray 2" "gray 2" and "black", which consumes more toner than the above example, the image data is expressed as "00101011" and the bit count is the same as before, i.e., 4. Thus, the toner consumption cannot be predicted correctly with this method.
In order to avoid such a shortcoming, an actual digital image output unit receives the image data in parallel and in synchronism with a pixel clock, and sets a threshold between, e.g., "gray 1" and "gray 2" to count the number of pixels whose densities excess this threshold However, the introduction of such a threshold parameter results in counting both pixels of "gray 2" and "black" as the same value, although the amount of toner consumed by them is different. Thus, this method is not satisfactory in performing accurate toner density control.